Changes in the Structure and the Thermal Properties of Kraft Lignin during Its Dissolution in Cholinium Ionic Liquids

Lignin valorization using ionic liquids (ILs) has attracted growing interest recently. Cholinium ILs, a novel kind of bio-ILs, are good lignin solvents. In this work, the changes in the structure and the thermal properties of kraft lignin during its dissolution in this type of bio-ILs were examined in detail by nuclear magnetic resonance spectroscopy, gel permeation chromatography, elemental analysis, thermogravimetric analysis, and differential scanning calorimetry. It was found that the β-O-4′ linkage was broken during kraft lignin dissolution, while the β-β′ and β-5′ linkages were formed. In addition, the dehydration and demethoxylation reactions occurred. Besides, the dissolution process resulted in a higher maximal decomposition temperature ( T m ) and a higher glass transition temperature ( T g ) of kraft lignin as well as the low residual char and the reduced molecular weights. Hence, these chemical transformations should be taken into account during lignin dissolution in ILs when the mechanism of lignin valorization using IL processes is proposed in the presence of catalysts.